The immune system plays a central role in the body's defense against cancer. Cellular immunity against tumors can be demonstrated in part because the ability of T-lymphocytes to engage in cell-mediated attack against tumor cells is made possible by the interaction between the T-lymphocyte receptor ("TCR": T-cell receptor) and specific class 1 HLA tumor cell markers (i.e., human leukocyte antigens: "HLA" molecules) present on the surface of the tumor cell. The TCRs on T-lymphocytes recognize a tumor antigen when the tumor antigen is associated with the HLA surface cell molecules. In particular, CD8 T-lymphocytes recognize antigens made within, or otherwise introduced into, the cytoplasm of the cell. Antigens in the cells are cleaved by cytoplasmic proteases into peptide fragments. The fragments are transported by an active transport process mediated by molecules encoded in the HLA itself into the endoplasmic reticulum where the fragments bind to newly made class I molecules. The peptide fragment is transported out to the cell surface where the T-lymphocytes recognize the combination of peptide and HLA. The peptide binds in the groove of the HLA molecule between two alpha helices that form the sides of the groove.
Through this recognition of presented surface tumor antigen (i.e., tumor-associated antigen: "TAA"), the T-lymphocytes are activated, expanded by clonal selection, and matured to give rise to tumor-specific, cytotoxic T-effector cells ("cytotoxic T-lymphocytes") together with an enlarged population of T-memory cells. Unfortunately, tumor cells are not especially effective in mediating strong immune responses.
The identity of the TAA's themselves is generally unknown. Recently, TAA's have been investigated in two different tumor systems. It has been demonstrated that T-lymphocytes recognize specific TAA presented by HLA moieties on the surface of melanoma cells; the so-called MAGE system. See for example, Kawakami et al., J. Immunol., 148:638, (1992). The MAGE system is, however, expressed in only 40-50% of these tumors which make up only about 1 percent of all cancers. The MZ2-E antigenic peptide of the MAGE system is presented by HLA-A1 molecules which are found in only 26% of Caucasians. Specific recognition of ovarian cancer TAA by T-lymphocytes has also been demonstrated. See Ioannides et al., Cell Immunol. 151:225-234(1993), incorporated herein by reference.
A source of the ovarian cancer TAA may be fragments of oncogene protein products. HER2/neu (also called c-erbB-2) is a 185 kDa transmembrane glycoprotein with tyrosine kinase activity and extensive homology to epidermal growth factor. HER2/neu is a ubiquitous oncogene which is expressed in normal cells (Coussens et al., Science, 230:1132) and overexpressed in about 30% of all ovarian and breast cancers. Overexpression of the HER2/neu oncogene correlates with a poor prognosis in these cancers. See Slamon et al., U.S. Pat. No. 4,968,603 (Nov. 6, 1990), incorporated herein by reference.
The identity of specific antigenic peptide fragments which are the target of T-lymphocyte recognition would allow peptides to be used to stimulate T-lymphocytes in vitro for specific adoptive immunotherapy.
Nonetheless, although HER2/neu is widely expressed in epithelial tumors such as ovarian, breast, lung, gastric, pancreatic, and colon (see, for example, Yamanaka et al., Human Pathology, 24:1127-1134(1993); Kern et al., Am. J. Resp. Cell Mol. Biol. 9:448-454(1993)), only peptide fragments based on HER2/neu proteins from normal cells have heretofore been shown to provide recognition for cytotoxic T-lymphocytes. See Ioannides et al., Abstract FASEB Meeting, 1992 page A1404. Furthermore, because of their amino acid sequence and size, these "normal" HER2/neu peptide fragments are not actually predicted to bind with HLA-A2 molecules on the tumor surface.
This is unfortunate because vaccines and immunotherapies based upon antigenic HER2/neu peptides ideally should be tumor-specific so that cytotoxic T-lymphocytes (CTL's) will consistently attack only the tumor, not normal tissue. Thus, truly useful antigenic peptides should be capable of initiating immune responses against tumor cells only.